1. Field of the Invention
This invention relates to an electric power steering apparatus.
2. Related Background Art
For vehicles and the like, there is known an electric power steering apparatus which detects by means of a torque sensor the steering torque generated in the steering system, and outputs an auxiliary torque to the steering system in accordance with the detected value by driving an electric motor connected to the steering system.
Among such electric power steering apparatuses, some of them are arranged to position the rotational shaft of an electric motor in the direction rectangular to the output shaft of the steering system due to the limited space or the like for the motor to be installed. In an electric power steering apparatus of the kind, the rotational shaft of the electric motor and the output shaft of the steering system are coupled by use of a bevel gear or highpoid gear mechanism in order to transmit the driving power while reducing the speed.
In the electric power steering apparatus described above, one problem is encountered that the backlash must be set appropriately between the engaging gears in the bevel gear or highpoid gear mechanism. If the backlash is too small, the engaging gears are caused to compete with each other. On the other hand, if the backlash is too large, the surfaces of the engaging gears are caused to collide with each other due to the abrupt variation of torque. As a result, there is a possibility that not only the hammering noises are generated, but also, the tooth of the gears is damaged, thus making it difficult to transmit the torque smoothly.
In the conventional technique, therefore, the structure is designed for a gear mechanism of the kind to adjust the backlash by installing the gears on one side in the bevel gear or highpoid gear mechanism to a shaft member, while supporting these gears by means of bearings to make them correlatively movable with respect to the gears on the other side by use of spacers of a given thickness provided to intervene between the shaft member and the bearings. However, according to the structure thus designed, the assembly is carried out by measuring the dimensions before the assembly so that the backlash can be estimated or the backlash must be measured once after the assembly in order to make the adjustment by trial and error by replacing spacers again. It takes a considerable time and labor to execute these types of work.
In the meantime, there is a structure disclosed in Japanese Patent Laid-Open Application No. 4-5168 whereby to adjust the backlash in such a manner that the gears on one side in the bevel gear or highpoid gear mechanism are mounted on the shaft member, and supported by means of bearings to make them correlatively movable with respect to the gears on the other side, and then, the position of the bearings in the axial direction is adjusted by means of adjusting plugs.
According to this structure, it is possible to adjust the backlash from the outside after the assembly by use of the adjusting plugs, but the movement of the shaft member in the axial direction is suppressed just by one bearing. As a result, when an excessive torque is transmitted, there is a possibility that the shaft member can move greatly in the axial direction within a range of play existing in the bearing. Thus depending on the magnitude of the power of an auxiliary steering, an increased backlash is invited, leading to the creation of the same problem as described above. Also, if a smaller backlash is set at the time of assembly, there is encountered a problem that the backlash becomes too small at the time of usual operation.
Further, in the highpoid gears and the like, both the driving and driven gears are conically formed. Thus the engaging relationship between them is unique such as its axes being orthogonal to or offset with each other. Therefore, it is a common practice that the driving gear is supported by a cantilever bearing.
However, in the electric power steering apparatus according to the conventional technique described above, the driving pear is supported by a cantilever bearing. Therefore, when a load is given to pears, a moment is generated in the bending direction of the shaft on which the driving gear is mounted. Thus the bearing is allowed to function as a fulcrum while the engaging section of the gears becomes the point of application. As a result, due to this bending of the shaft, the abutting positions of the gear tooth are caused to shift, thereby to invite the deviated abrasion of the gear tooth and the increased noises of gear engagement as well. Meanwhile, the driven gear is caused by the shaft of the driving gear thus bent to displace its engaging tooth away from the tooth of the driving gear, hence inviting the increased backlash. In some cases, the driven gear itself is caused to shift in the bending direction of the shaft.
Meanwhile, in an electric power steering apparatus of the kind, a detector is provided in a housing for detecting the steering power exerted on the input shaft so that the electric motor can be controlled.
Now, for the power transmission mechanism made of iron or the like, a lubricant suitable for use of a low-speed and heavy load must be applied in order to reduce the friction and the like. On the other hand, for the detector, a lubricant suitable for use of a high-speed and light load must be applied. However, since these two members are arranged in one and the same housing, there is a fear that if these two different kinds of lubricants are used, a mixture takes place during operation, thus lowering the performance of these lubricants. Also, there is a fear that an adverse effect is produced on the function of the detector by the iron particles generated when the gears engage with each other in the power transmission mechanism. Therefore, it is arranged in a certain type of the conventional electric power steering apparatus that at least gears on one side in the power transmission mechanism are made of resin so that the mechanism is operative even in a state where no lubricant is applied. A measure is also taken to prevent the generation of iron particles. However, the power transmission mechanism of this type cannot transmit a high torque. Therefore, it is desired to provide an electric power steering apparatus having a power transmission mechanism capable of transmitting a higher torque.